For example, Patent Document 1 discloses a conventional valve device including a ball valve and a valve seat. The ball valve has an internal passage therein and rotatable around a predetermined rotation axis thereby to change the direction of the internal passage. The valve seat is substantially in a tubular shape extending in an axial direction, which is different from an axial direction of the rotation axis. The valve seat forms a passage, which is communicated with the internal passage and is blocked from the internal passage. The valve seat has one end in the axial direction, and the one end has an opening surrounded by a concave spherical surface. The ball valve is slidable and rotatable on the spherical surface. The valve seat has the other end in the axial direction, and the other end is equipped with a spring. The spring depresses the valve seat toward the ball valve.
(Patent Document 1)
Publication of an unexamined German patent application No. 10 2009 014 047
A ball valve may be deviated from a predetermined position due to various reasons, such as a variation in a component dimension, deformation caused in the ball valve when applied with a pressure, and/or deformation caused in the shaft to which the ball valve is fixed. Consequently, the ball valve may be displaced in the axial direction of the rotation axis and/or in a direction perpendicular to the axial direction. The displacement in the direction perpendicular to the axial direction may be absorbed to some degree by expansion and contraction of the spring to move the position of the valve seat. It is noted that, the displacement in the axial direction may not be sufficiently absorbed by the spring.
When the displacement of the ball valve arises, the spherical surface of the ball valve may not fit to the concave spherical surface of the valve seat. Consequently, fluid may leak through a gap between the ball valve and the valve seat. In addition, the deviation in the position of the ball valve may cause inclination of the valve seat. Consequently, the spring may apply a biasing force eccentrically to the valve seat. Thus, the spring cannot apply the biasing force uniformly to the periphery of the opening of the passage sufficiently to seal between the valve seat and the ball valve. As a result, fluid may leak through a gap between the ball valve and the valve seat.